1. Field of the Invention
The invention relates to a diffusion barrier for reducing the diffusion of copper features, and more particularly to a copper alloy diffusion barrier between copper features and an oxygen containing polymer in an electronic component.
2. Description of Related Art
Integrated circuit chips are normally packaged and interconnected to one another in order to provide a functional electronic computer system. Packaging and interconnection can be accomplished by surface mounting the chips on high performance high density multilayer interconnect (HDMI) substrates (or boards) for multichip modules (MCM). Such substrates not infrequently contain copper features (lines and vias) buried in an oxygen containing polymer dielectric. Normally there are several requirements for the materials used as interlayer dielectrics in HDMI substrates. There include low dielectric constant, low dielectric loss, low thermal coefficient of expansion (TCE), ease of processing, good adhesion to copper and polyimide, and minimal reactivity with other materials. Achieving all of these properties in any one material is difficult. The most popular materials in use at the present time are various polyimides due to their electrical properties and also their low TCE. A major disadvantage of polyimides is their reactivity with copper, which is the most popular metal used for electrical conductors. Copper diffuses into polyimide during high temperature processing of the polyimide. This causes severe corrosion of the copper and the polyimide due to the copper combining with oxygen in the polyimide. The corrosion may result in loss of adhesion, delamination, voids, and ultimately a catastrophic failure of the HDMI substrate.
Other methods have formerly been developed in order to solve the corrosion problem. One approach is to introduce a diffusion barrier between the copper and the polyimide. This diffusion barrier could be metallic, organic, or an inorganic. Metal diffusion barriers have been provided by nickel, titanium, and chromium. For instance, in an additive process disclosed in U.S. Pat. No. 4,810,332 by Pan, the copper pillars are electroplated with 7,000 angstroms of nickel. Thereafter, conventional polyimide deposition and polishing for planarization is applied. The nickel overcoat was demonstrated to provide excellent protection for copper against oxidation/Corrosion during a 390.degree. C. curing step with polyimide over the nickel overcoat. Unfortunately, several drawbacks may arise from selectively electroplating nickel over copper features, including uncoated copper features and "sheeting" nickel on polyimide between spaced copper features. These drawbacks may to occur due to undesired humidity and/or contaminants. Another approach is used in substractive processes where either titanium or chromium is sputtered onto copper, and this thin metal layer is used to stop the copper to polyimide diffusion. Drawbacks with substractive processes may include the need for an additive process, and insufficient resolution from a mask for fine (20 micron) copper features.
Perhaps the ideal solution to the diffusion problem would be the development of a polymer which prevents copper diffusion while fulfilling all the thermal, electrical, and mechanical requirements. Unfortunately, Applicant does not foresee the advent of a new polymer dielectric which will satisfy these requirements in the near future. Therefore, any overcoat method which reduces the complexity of providing a diffusion barrier for copper features adjacent oxygen containing polymers in electronic components may be useful in reducing fabrication costs and enhancing reliability.